The Series 90 definitely is, with a Wimbledon victory bona fide and its rarity behind it. The Series 125 was rare and weird. The 110 is easier to find, more accessible and probably ought to be considered the high-water mark in metal racquets, but it’s critical you find one whose head isn’t deformed from excessive wear or abuse. If it’s in good condition, $20 is a great price.
I would say $20 is an ok price. It's not a collectible and probably won't ever be. I had one about 5 years ago and it wasn't anything special. I found a Mag 90 at about the same time and that hit great.
If you want it to hit with, go pick it up. If you are looking to make some money - pass.